I think that it is in how your are forming the file path. I am by no means an apple script guru but I tend to use the mac friendly form in which case it would look something like "DriveName:Volumes:Test_LowRes".

Set that to a variable as an alias and then use that variable in the line - set x to filenames of variable

I would highly recommend using the Script Master plugin for this if possible. The ListAllFilesInDirectory function returns a value list of the files that you can then do with as you like. Speaking from experience it is a heck of a lot easier than going "outside" filemaker.

My code looks like this and results in the field "gdestination_folder_contents" having a list of file names from the "Shared" folder on my hard drive. I haven't tried it for a mounted volume. However, I think it will work the same.

tell application "Finder"

set oldDelimiters to AppleScript's text item delimiters

set AppleScript's text item delimiters to return

set gdestfile to name of entire contents of folder "my_computer:Users:Shared:" as text